Substrate use and its effect on body temperature in two syntopic <em>Liolaemus</em> lizards in northwestern Argentina
Habitat use and thermal biology are closely related, because thermal microclimates vary spatially. The use of habitat and microhabitat by different species influences many of their traits, such as their physiology, and may, therefore exert a direct effect on survival. Ectothermal animals, such as lizards, are affected by thermal and biophysical environments they inhabit, and the particular use of a given substrate reflects an overlap between thermally adequate microhabitats, and behavioral preferences. By exploiting certain microhabitats and avoiding others, many lizards tend to maintain their body temperature within a range that allows maximum performance. Here, we evaluate how two syntopic species of lizards, Liolaemus pacha and L. ramirezae, use substrates with different exposure to solar radiation. Our hypothesis is that L. pacha uses both soil and rock substrates indistinctly, due to being a generalist species, whereas L. ramirezae uses the rock substrate more frequently, due to its saxicolous habits. We expect temperatures to be different both in substrates, and in different exposures, and thermal characteristics of each species to condition their use. For example, because the body temperature range of L. pacha is wider, we predict that substrate use will be wider. A pre-established 100x75 m area was monitored during four Austral springs and summers between 2011 and 2015, in Los Cardones, Amaicha del Valle, Tucumán, Argentina. Species' substrate where the lizard was found (soil or rock), and exposure to solar radiation: sun, filtered shade or full shade was recorded. After capture, lizard body temperature (Tb), substrate temperature (Ts), and air temperature (Ta) were recorded in the place of the first observation of the lizard. Obtained results show that L. pacha and L. ramirezae had a more persistent use of the rock than the soil substrate, thus considering them saxicolous species. Further, they were frequently observed exposed to direct sunlight. Average body temperature was higher than environmental temperature (Ts and Ta), and significantly different in each exposure type (sun, filtered shade and full shade), and in both substrates (rock and soil). Differential use of substrate and the relationship between body temperature and microhabitat temperatures suggests that L. pacha and L. ramirezae are “active thermoregulators”, using both substrate surfaces and solar radiation as heat sources.